The present invention relates to a data transmission device for transmitting data between microprocessors and/or computers and related equipment and more particularly relates to an interface between data communications equipment (DCE) and data terminal equipment (DTE).
The DTE-DCE interface which is currently most commonly in use to interconnect I/O ports in serial communications systems in the United States conforms to the RS-232C standard as published by the Electronics Industry Association (EIA). The interface is commonly referred to as the EIA RS-232C interface or simply as the RS-232C interface. Because of the nature of the RS-232C interface there are practical limits to the distance over which it may be used without requiring additional equipment. For example, at 110 baud, RS-232C transmissions are generally limited to approximately 1000 feet. At 9600 baud, they are generally limited to 50 feet. Under the EIA standard the recommended maximum cable length is 50 feet.
The RS-232C and the like standards are intended to define the interface between DTE's and DCE's for communications over networks, usually telephone facilities. They actually do not directly address the use of the interface as a general serial I/O port nor the connecting of peripheral devices such as printers to DTE's such as computers. Nevertheless this type of usage of the RS-232C interface is common and the foregoing distance limitations generally present no serious problems since typical connections are frequently 15 feet or shorter. On the other hand occasions frequently arise when it is desired to transmit data communications over longer distances at the same site without utilizing telephone lines or the like.
At the present time it is common practice to cope with this problem through the use of devices which are commonly known as modem eliminators. Generally speaking these devices constitute autonomous self-contained A.C. line powered units which interconnect computers and/or microprocessor ports to a terminal within the parameters of EIA Specification RS-232C. The units usually provide operation at multiple bps speeds which are switch selectable. While modem eliminators of this type are generally effective for their intended purposes they are relatively expensive and involve the use of additional separately housed equipment connected to the A.C. power mains.